U.S. patent number 6,819,812 [Application Number 10/412,671] was granted by the patent office on 2004-11-16 for system and method for measuring physical, chemical and biological stimuli using vertical cavity surface emitting lasers with integrated tuner.
This patent grant is currently assigned to Lake Shore Cryotronics, Inc.. Invention is credited to Vladimir Kochergin, Philip Swinehart.
United States Patent |
6,819,812 |
Kochergin , et al. |
November 16, 2004 |
System and method for measuring physical, chemical and biological
stimuli using vertical cavity surface emitting lasers with
integrated tuner
Abstract
An optical sensor diagnostic system uses a tunable Vertical
Cavity Surface Emitting Laser (VCSEL) that incorporates an
integrated MEMS tuning mechanism. The system provide variable
wavelength light into an optical fiber with improved wavelength
scanning speed and greater simplicity of construction. Sensors,
such as Bragg gratings, are disposed along the fiber in the light
path. Each sensor reflects or transmits light exhibiting a
characteristic amplitude feature with respect to wavelength, the
wavelength position of which is affected by an environmental
stimulus imposed thereon. The reflected light is converted to an
electrical signal by a simple detector and monitored by circuitry
that detects changes. The system provides output signals indicative
of the environmental stimulus for each sensor.
Inventors: |
Kochergin; Vladimir
(Westerville, OH), Swinehart; Philip (Columbus, OH) |
Assignee: |
Lake Shore Cryotronics, Inc.
(Westerville, OH)
|
Family
ID: |
25530228 |
Appl.
No.: |
10/412,671 |
Filed: |
April 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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983999 |
Oct 26, 2001 |
6549687 |
|
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Current U.S.
Class: |
385/12;
372/20 |
Current CPC
Class: |
G02B
26/001 (20130101); G01N 21/39 (20130101); G01N
21/45 (20130101); G01N 21/553 (20130101); G01N
21/7703 (20130101); G02B 6/42 (20130101); H01S
5/18366 (20130101); G02B 6/4206 (20130101); G01N
2021/399 (20130101); G02B 6/29358 (20130101); G02B
6/4202 (20130101); G01N 21/64 (20130101) |
Current International
Class: |
G01B
9/02 (20060101); G01N 21/45 (20060101); G01N
21/31 (20060101); G01N 21/39 (20060101); G02B
26/00 (20060101); G01N 21/41 (20060101); G02B
6/42 (20060101); G01N 21/64 (20060101); G01N
21/55 (20060101); G02B 6/34 (20060101); H01S
5/00 (20060101); H01S 5/183 (20060101); G02B
006/26 (); H01S 003/10 () |
Field of
Search: |
;385/12,18,24
;372/20 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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signals in a fiber Brag grating sensor using a tunable laser and a
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|
Primary Examiner: Lee; John D.
Assistant Examiner: Lin; Tina M
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of application Ser. No.
09/983,999, filed Oct. 26, 2001, now U.S. Pat. No. 6,549,687, which
is hereby incorporated by reference.
Claims
We claim:
1. An optical sensor diagnostic system, comprising: a tunable VCSEL
incorporating an integrated MEMS wavelength tuner for providing
wavelength-tunable light in response to a tuning control signal,
said tunable light being launched into an optical waveguide, said
tunable VCSEL including a movable tuning mirror and at least one
optical, capacitive, piezoelectric or magnetic detector for
detecting the position of the movable tuning mirror and providing
feedback; at least one optical sensor, disposed in the path of said
tunable light, said at least one sensor providing a transmitted
light having at least one associated characteristic amplitude
feature selected from the group consisting of a minimum, a maximum
or a slope located at a particular wavelength within the
transmitted wavelength range, said wavelength at each minimum,
maximum or sloped transmission amplitude being responsive to an
environmental stimulus imposed upon said at least one sensor; said
tunable VCSEL individually illuminating said at least one sensor in
a wavelength range spanning said wavelength location of said
associated characteristic transmission amplitude feature; an
optical isolator, disposed in the path of said tunable light
between said tunable VCSEL and said at least one sensor, said
optical isolator isolating said tunable light source from light
reflected from said at least one sensor; an optical detector
disposed in the path of said transmitted light, said optical
detector detecting said transmitted light from said at least one
sensor and providing an electrical detection signal indicative of
the power of said transmitted light throughout a predetermined
wavelength range; a controller for providing a variable tuning
control signal to said tunable VCSEL indicative of the desired
wavelength of said tunable light; at least one wavelength reference
independent of said tuning control signal and moveable mirror
position detector disposed in the path of the light; and a signal
processor responsive to said electrical detection signal, for
detecting a wavelength defined on the characteristic transmission
amplitude feature in order to quantitatively detect the effect on
said at least one sensor due to said environmental stimulus,
changes in said wavelength at the characteristic transmission
amplitude feature caused by changes in said environmental stimulus,
and for providing a signal indicative of said stimulus or change
therein.
2. The optical sensor diagnostic system of claim 1 further
including an optical amplifier disposed between said tunable VCSEL
and said at least one sensor.
3. The optical sensor diagnostic system of claim 2 wherein said
optical amplifier comprises an Erbium doped fiber amplifier.
4. The optical sensor diagnostic system of claim 2 wherein said
optical amplifier comprises a semiconductor optical amplifier.
5. The optical sensor diagnostic system of claim 1 wherein said
tunable VCSEL includes an optical amplifier.
6. The optical sensor diagnostic system of claim 5 wherein said
optical amplifier comprises an Erbium doped fiber amplifier.
7. The optical sensor diagnostic system of claim 5 wherein said
optical amplifier comprises a semiconductor optical amplifier.
8. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises plural wavelength division multiplexed
optical sensors.
9. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises plural time division multiplexed optical
sensors.
10. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises an environment reference or compensation
sensor.
11. The optical sensor diagnostic system of claim 1 wherein said
detection and signal processor comprises a tracker, responsive to
said electrical detection signal, for adjusting a voltage or other
tuning control signal to allow said tunable light to track static
and dynamic values of said characteristic transmission amplitude
feature for said at least one sensor, thereby providing utilization
of the control signal as the output characteristic of the physical
stimulus, making unnecessary scanning of the complete wavelength
range and greatly increasing the speed of data acquisition.
12. The optical sensor diagnostic system of claim 1 wherein said
controller comprises a modulator for modulating said voltage
control signal at a predetermined modulation frequency.
13. The optical sensor diagnostic system of claim 1 wherein said
signal processor comprises a demodulator operating at said
modulation frequency, for demodulating said electrical detection
signal and for providing a demodulated signal indicative
thereof.
14. The optical sensor diagnostic system of claim 1 wherein said
signal processor incorporates a computational element for
increasing the accuracy and precision of determining the wavelength
position of said characteristic transmission amplitude feature and
changes therein for each of said sensors.
15. The optical sensor diagnostic system of claim 1 wherein: said
at least one sensor comprises plural sensors; said controller
comprises a scanner that scans said control signal for the purpose
of causing said tunable VCSEL to scan its wavelengths across said
characteristic transmission amplitude features of said plural
sensors; and said signal processor determines, in response to said
voltage or other control signal, the wavelength of said tunable
light from the magnitude of said voltage or other control signal
and/or mirror position feedback signal and for determining which of
said plural sensors is being illuminated, thereby determining the
value of the environmental stimulus at the position of said
illuminated sensor.
16. The optical sensor diagnostic system of claim 1 wherein: said
at least one sensor comprises plural sensors; said controller
comprises a scanner that scans said control signal so as to cause
said tunable VCSEL to scan across the characteristic transmission
amplitude features of said plural sensors and for providing a
synchronization signal indicative of when said voltage control
signal begins and ends said scanning; and said signal processor
determines, in response to said synchronization signal, which of
said plural sensors is being illuminated, thereby determining
changes in said wavelength at said characteristic transmission
amplitude feature.
17. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one fiber or planar Bragg
grating, narrow bandpass filter or long period fiber Bragg
grating.
18. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises a long period fiber Bragg grating having
a period exceeding 10 micrometers.
19. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor includes at least one Bragg grating that
incorporates phase shift in its structure, said phase shift
producing a sharper maximum within said transmitted wavelength band
minimum.
20. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one Fabry-Perot etalon.
21. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one Surface Plasmon Resonance
structure.
22. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one thin film or bulk material
characteristic absorber material.
23. The system of claim 22 wherein characteristic absorber material
comprises one or more vibronic, excitonic or fluorescent
materials.
24. The system of claim 22 wherein said characteristic absorber
material comprises at least one semiconductor.
25. The system of claim 22 wherein said characteristic absorber
material is chosen from the group consisting of alloys and
compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead,
aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus,
arsenic, antimony, sulfur, selenium and tellurium.
26. The system of claim 22 wherein said sensor produces a
characteristic absorption feature in the form of a slope, and
wherein the said wavelength indicative of the characteristic
absorption slope is determined by taking the first derivative of
the light amplitude with respect to the wavelength, and by
analytically extracting the wavelength position resulting from said
first wavelength derivative extremum.
27. The system of claim 22 wherein said sensor produces a
characteristic absorption feature in the form of a slope, and
wherein the said wavelength indicative of the characteristic
absorption slope is determined by taking the second derivative of
the light amplitude with respect to the wavelength, and by
analytically extracting the wavelength positions of said second
wavelength derivative zeros.
28. The optical sensor diagnostic system of claim 1 wherein said
environmental stimulus comprises any combination of mechanical
stress, temperature, pressure, electrical current, electrical
field, magnetic field or chemical or biological material on said
sensor.
29. The optical sensor diagnostic system of claim 1 wherein at
least one wavelength reference, not affected by any environmental
stimulus, comprising at least one of the group of a Bragg grating,
a phase shifted Bragg grating, a Fabry-Perot etalon or a
gas-containing chamber, at least one narrow bandpass filter or a
gas-containing chamber, is disposed in the optical path.
30. The optical sensor diagnostic system of claim 29 wherein the
wavelength reference comprises at least one gas-containing chamber
containing acetylene gas.
31. An optical sensor diagnostic system, comprising: a VCSEL
incorporating integrated MEMS wavelength tuner for providing
wavelength-tunable light in response to a tuning control signal,
said tunable light being launched into an optical waveguide,
wherein is provided at least one optical sensor, disposed in the
path of said tunable light, each providing a reflected light having
at least one associated characteristic amplitude feature selected
from the group, a minimum, a maximum or a slope located at a
particular wavelength within the reflected wavelength range, said
wavelength at each minimum, maximum or sloped reflection amplitude
being responsive to an environmental stimulus imposed upon a
corresponding sensor; said tunable VCSEL for individually
illuminating each of said sensors in a wavelength range spanning
said wavelength location of said associated characteristic
reflection amplitude feature; optical detector, disposed in the
path of said reflected light, for detecting said reflected light
from each of said sensors and for providing an electrical detection
signal indicative of the power of said reflected light throughout
the appropriate wavelength range; an arrangement including at least
one of an optical circulator and an optical splitter, said optical
circulator or optical splitter being optically coupled to with an
optical isolator, said arrangement being disposed in the path of
said tunable light between said tunable VCSEL and said sensor, for
isolating said tunable light source from light reflected from said
sensor and directing the light to said detector; voltage or other
controller for providing a variable tuning control signal to said
tunable VCSEL indicative of the desired wavelength of said tunable
light; at least one capacitive, optical, piezoelectric or magnetic
detector that detects the position of the movable tuning mirror and
providing feedback; and at least one wavelength reference
independent of said tuning control signal and moveable mirror
position detector disposed in the path of the light, and signal
processor responsive to said electrical detection signal, for
detecting a wavelength defined on the characteristic reflection
amplitude feature in order to quantitatively detect the effect on
said sensor due to said environmental stimulus, changes in said
wavelength at the characteristic reflection amplitude feature
caused by changes in said environmental stimulus, and for providing
a signal indicative of said stimulus or change therein for each of
said sensors.
32. The optical sensor diagnostic system of claim 31 wherein said
optical sensors are wavelength division multiplexed.
33. The optical sensor diagnostic system of claim 31 wherein said
optical sensors are time division multiplexed.
34. The optical sensor diagnostic system of claim 31 wherein at
least one of said sensors serves as an environment reference or
compensation sensor.
35. The optical sensor diagnostic system of claim 34 wherein said
detection and signal processor comprises a tracking means,
responsive to said electrical detection signal, for adjusting said
voltage or other control signal to allow said tunable light to
track static and dynamic values of said characteristic reflection
amplitude feature for each of said sensors, thereby providing
utilization of the control signal as the output characteristic of
the physical stimulus, making unnecessary scanning of the complete
wavelength range and greatly increasing the speed of data
acquisition.
36. The optical sensor diagnostic system of claim 31 wherein said
voltage controller comprises a modulator for modulating said
voltage control signal at a predetermined modulation frequency.
37. The optical sensor diagnostic system of claim 31 wherein said
signal processor comprises a demodulator operating at said
modulation frequency, for demodulating said electrical detection
signal and for providing a demodulated signal indicative
thereof.
38. The optical sensor diagnostic system of claim 31 wherein said
signal processor performs computations that increase the accuracy
and precision of determining the wavelength position of said
characteristic reflection amplitude feature and changes therein for
each of said sensors.
39. The optical sensor diagnostic system of claim 31 wherein: said
voltage or other controller comprises a scanner that scans said
control signal so as to cause said tunable VCSEL to scan its
wavelength across said characteristic reflection amplitude feature
of any or all of said sensors; and said signal processor
determines, in response to said voltage or other control signal,
the wavelength of said tunable light from the magnitude of said
voltage or other control signal and/or mirror position feedback
signal and for determining which of said sensor is being
illuminated, thereby determining the value of the environmental
stimulus at the position of said individual sensor.
40. The optical sensor diagnostic system of claim 31 wherein: said
voltage or other controller comprises a scanner that scans said
voltage control signal so as to cause said tunable VCSEL to scan
across the wavelengths of the characteristic reflection features of
all of said sensors; and for providing a synchronization signal
indicative of when said voltage control signal begins said
scanning; and said signal processor determines, in response to said
synchronization signal, which of said sensors is being illuminated,
thereby determining changes in said wavelength at said
characteristic reflection amplitude feature.
41. The optical sensor diagnostic system of claim 31 wherein said
at least one sensor comprises at least one fiber or planar Bragg
grating.
42. The optical sensor diagnostic system of claim 41 wherein at
least one Bragg grating of at least one sensor comprises at least
one incorporated phase shift in its structure, said phase shift
producing a sharper minimum within said reflected wavelength band
maximum.
43. The optical sensor diagnostic system of claim 31 wherein said
at least one sensor comprises at least one Fabry-Perot etalon.
44. The optical sensor diagnostic system of claim 31 wherein said
at least one sensor comprises at least one Surface Plasmon
Resonance structure.
45. The optical sensor diagnostic system of claim 31 wherein at
least one sensor is disposed in a branch waveguide or optical fiber
coupled to the main trunk waveguide by a coupler.
46. The optical sensor diagnostic system of claim 31 wherein said
at least one sensor comprises at least one thin film or bulk
material characteristic absorber material.
47. The characteristic absorber material of claim 46 comprising at
least one semiconductor.
48. The characteristic absorber material of claim 47 chosen from
the full possible range of alloys and compounds of zinc, cadmium,
mercury, silicon, germanium, tin, lead, aluminum, gallium, indium,
bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur,
selenium and tellurium.
49. The characteristic absorber material of claim 46 comprises one
or more vibronic, excitonic or fluorescent materials.
50. The characteristic absorber material of claim 46 wherein said
sensor comprised of said characteristic absorber material
incorporates a mirror at the distal end, providing signal
reflection by double-pass transmission.
51. The optical sensor diagnostic system of claim 31 wherein at
least one sensor produces a characteristic absorption feature in
the form of a slope, wherein: the said wavelength indicative of the
characteristic absorption slope is determined by taking the first
derivative of the light amplitude with respect to the wavelength,
and by analytically extracting the wavelength position of resulting
said first wavelength derivative extremum, or, alternatively, the
said wavelength indicative of the characteristic absorption slope
is determined by taking the second derivative of the light
amplitude with respect to the wavelength and by analytically
extracting the wavelength positions of said second wavelength
derivative zeros.
52. The optical sensor diagnostic system of claim 31 wherein said
environmental stimulus is any combination of mechanical stress,
temperature, pressure, electrical current, electrical field,
magnetic field or chemical or biological material on said
sensor.
53. The optical sensor diagnostic system of claim 31 wherein at
least one wavelength reference, not affected by any environmental
stimulus, comprising at least one of the group of a Bragg grating,
a phase shift Bragg grating, a Fabry-Perot etalon or a
gas-containing chamber, is disposed in the optical path.
54. The optical sensor diagnostic system of claim 53 wherein the
gas-containing chamber contains acetylene gas.
55. The optical sensor diagnostic system of claim 31 wherein a said
sensor comprising an optical fiber having a core waveguide and a
cladding or cladding/buffer layer surrounding the core waveguide,
in addition incorporating an input/output end and a terminal
reflection end, wherein the terminal reflection end is defined by
an end face of the core waveguide in contact with a mirrored layer
such that the light is caused to reverse its direction of
propagation and exits the input/output end, said sensing area is
defined by a surface plasmon resonance-supporting metal in contact
with at least a portion of the surface of the optical fiber core
waveguide free from the surrounding cladding or cladding/buffer
layer.
56. The optical sensor diagnostic system of claim 44 wherein the
said sensing area further contains at least one additional
functional layer adhered to the surface plasmon
resonance-supporting metal layer.
57. The optical sensor diagnostic system of claim 56 wherein the at
least one additional layer comprises a chemically reactive
layer.
58. The optical sensor diagnostic system of claim 56 wherein the at
least one additional layer comprises a biologic ally reactive
layer.
59. The optical sensor diagnostic system of claim 55 wherein the
said plasmon resonance supporting metal is one or more layers of
elements or alloys chosen from the group consisting of silver,
gold, copper, aluminum, indium or palladium.
60. The optical fiber sensor according to claim 55 further
incorporating a polarizer positioned anywhere between said tunable
VCSEL and said sensor, said polarizer selecting light with
polarization P.
61. The optical fiber sensor according to claim 56 further
comprising a first polarizer positioned between the said tunable
VCSEL and said circulator, said polarizer selecting light with
polarization state between S and P polarizations.
62. The optical fiber sensor according to claim 61 further
comprising a second polarizer positioned between the said sensor
and said detector, said second polarizer being oriented with
respect to the said first polarizer such that a phase-polarization
enhancement is obtained of the ratio of the power amplitudes at
wavelengths outside the said minimum of reflection feature to the
power amplitude at the exact minimum of reflection.
63. In a sensing system of the type including an optical fiber
sensor or optical waveguide having a radiation reflectance or
transmissivity characteristic that varies in response to a
stimulus, an optical path being defined between a coupler and said
optical fiber sensor or optical waveguide, a sensing method
comprising: (1) operating a Vertical Cavity Surface Emitting Laser
to generate radiation; (2) tuning the Vertical Cavity Surface
Emitting Laser to vary the wavelength of said Vertical Cavity
Surface Emitting Laser-generated radiation; (3) coupling at least
some of the radiation emitted by said Vertical Cavity Surface
Emitting Laser to said coupler; and (4) analyzing radiation
transmitted or reflected by said sensor or waveguide for variations
of said characteristic caused by said stimulus.
64. A sensing system including an optical fiber sensor or optical
waveguide, said system comprising: a coupler being coupled to the
optical fiber sensor or optical waveguide having a
wavelength-selective radiation transmissivity characteristic that
varies in response to a stimulus; a Vertical Cavity Surface
Emitting Laser coupled to said coupler, said Laser operated to
generate radiation and supply said radiation to said coupler; a
tuning device that tunes the Vertical Cavity Surface Emitting Laser
to vary the wavelength of said Vertical Cavity Surface Emitting
Laser-generated radiation; a detector that detects radiation
transmitted or reflected by the optical fiber sensor or waveguide;
and an analyzer that analyzes said detected radiation for at least
one variation caused by said stimulus.
65. A sensing system including an optical fiber sensor or optical
waveguide, said system comprising: coupling means being coupled to
the optical fiber sensor or optical waveguide having a
wavelength-selective radiation transmissivity characteristic that
varies in response to a stimulus; Vertical Cavity Surface Emitting
Laser means coupled to said coupler, said Laser means for
generating radiation and for supplying said radiation to said
coupler; tuning means for tuning the Vertical Cavity Surface
Emitting Laser means to vary the wavelength of said generated
radiation; detecting means for detecting radiation transmitted or
reflected by the optical fiber sensor or waveguide; and analyzing
means for analyzing said detected radiation for at least one
variation caused by said stimulus.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD
This invention relates to systems using vertical cavity, surface
emitting lasers (VCSELs) having integrated MEMS
(micro-electromechanical) wavelength tuners to interrogate optical
sensors such as fiber and planar Bragg gratings, etalons,
characteristic absorption or reflection sensors such as bandgap
semiconductors and surface plasmon resonance sensors sensitive to
physical, chemical and biological stimuli and, more particularly,
to specific system configurations for use with such Bragg grating,
etalon, absorption/reflection and surface plasmon resonance sensing
devices.
BACKGROUND AND SUMMARY
Fiber optic sensors employing measurements of the shift of
wavelength position of a sensor spectral peculiarity (e.g.,
maximum, minimum, slope or some other function) under the influence
of a physical stimulus are well known to those skilled in the art.
The examples of such sensors include Bragg grating-based strain,
pressure, temperature and current (via the associated magnetic
fields) sensors, surface plasmon resonance (SPR) biological and
chemical sensors, semiconductor absorption band-edge based
fiber-optic sensors and Fabry-Perot (FP) etalon pressure,
temperature and sensors. To date, the utilization of such sensors
has been retarded in the marketplace because of many well known
problems, including e.g. the susceptibility of simple, inexpensive
sensing systems to optical noise and the great expense of most of
the solutions found to overcome said susceptibility. It will be
revealed that combining a new type of laser--a vertical cavity,
surface emitting laser (VCSEL) with an integrated
microelectromechanical (MEMS) tuning mechanism--as an interrogating
instrument with sensors of many different types will enable new,
less expensive and more reliable classes of optical sensor
systems.
Generally, a Bragg grating is a series of optical elements that
create a periodic pattern of differing indices of refraction in the
direction of propagation of a light beam. A Bragg grating can be
formed in an optical fiber by exposing ultraviolet sensitive glass
(usually germanium doped fiber) with an ultraviolet (UV) beam that
varies periodically in intensity. This is usually accomplished by
means of an interference pattern created by a phase mask or split
beam, such as with a Lloyd's mirror apparatus. Planar Bragg
gratings are created by exposing a "photoresist" of any of a number
of types through a phase shift or other type of mask, or they can
be written directly with an electron beam. Light reflections caused
by the periodic index of refraction pattern in the resulting
grating interfere constructively and destructively. Since the
refractive index contrast between UV-exposed and unexposed sections
of fiber is small but the number of sections is very large, the
reflected beam narrows its spectrum to a very sharp peak--as narrow
as a fraction of a nanometer in spectral width. It can also be
arranged by means of a phase shift design that the reflected peak
can contain within it an even narrower "valley" of absorption, e.g.
as narrow as a few picometers in spectral width. Conversely, the
transmitted portion of the light beam exhibits complimentary
spectral power characteristics, i.e., a broader valley with a
narrower peak within it.
It is known that Bragg gratings patterned into optical fibers or
other waveguides may be used to detect physical stimuli caused by
various physical parameters, such as, for example, strain,
pressure, temperature, and current (via the associated magnetic
fields) at the location of the gratings. See e.g. U.S. Pat. Nos.
4,806,012 and 4,761,073 both to Meltz, et al; U.S. Pat. No.
5,380,995 issued to E.
Udd; U.S. Pat. No. 6,024,488 issued to J. Wu; and the publication
authored by Kersey, A. D., et.al. [10.sup.th Optical Fiber Sensors
Conference, Glasgow, October 1994, pp.53-56].
Generally, in one exemplary such sensor, the core and/or cladding
of the optical fiber (or planar waveguide) is written with periodic
grating patterns effective for selectively reflecting a narrow
wavelength band of light from a broader wavelength band launched
into the core (waveguide layer in the waveguide). The spectral
positions of sharp maxima or minima in the transmitted and
reflected light intensity spectra indicate the intensity of strain,
temperature, pressure, electrical current, or magnetic field
variations at the location of the grating. The spectral positions
change based on the grating period or the indices of refraction, or
both. These can be affected by various environmental physical
stimuli such as temperature and pressure.
Frequently, more than one stimulus or physical parameter affects
the sensors at the same time. Compensation is often designed into
the sensor or the measurement technique for all the variables but
one. This can be accomplished by many physical, optical and
electronic techniques known in the art. The typical sensitivity
limits of fiber grating sensors in the current art are about
0.1.degree. C. and/or 1 microstrain, respectively.
Exemplary advantages of a spectral shift method of sensor
interrogations include the high accuracy of wavelength
determination (akin to the advantages of measuring electrical
frequency instead of magnitude) and immunity to "optical noise"due
to fluctuations in fiber transmission amplitude (microbending
losses, etc.). The spectral shift method also allows the
multiplexing of many sensors on the same fiber via wavelength
dependent multiplexing techniques (WDM), e.g., dividing the total
wavelength band into sections dedicated to individual sensors.
The precision, dynamic range and multiplexing capabilities of the
optical sensor interrogation techniques can be defined in part by
the spectral power of the light source, especially in cases in
which a broadband source is used. The LEDs, SLDs (superluminescent
diodes) and various lamps usually used provide spectral power that
can be too small when divided into nanometer-sized segments. This
limits critical parameters such as the magnitude of the reflected
peak available to the optical sensor, causing lower than desirable
signal to noise ratios. Another technique, the use of a
conventional laser diode tuned with a motorized external cavity,
electrical current or temperature mechanisms is more effective
because all the power of the laser is contained in a narrow beam as
it is tuned across the spectrum. Several techniques have been
proposed: see for example Froggatt, (U.S. Pat. No. 5,798,521). The
use of a conventional laser diode tuned with electrical current has
been proposed by Dunphy et al. (U.S. Pat. No. 5,401,956); and the
use of a tunable fiber laser has been proposed by G. A. Ball et al.
[J. of Lightwave Technology, vol. 12, no. 4, April 1994 p.
700].
When using a scanning laser technique, an inexpensive detector and
electronics system can simply determine the wavelength at the peak
(or null) of the reflected (or transmitted) light intensity against
a known wavelength reference. However, past approaches are
generally too expensive, too slow, too unstable or too inaccurate
to have a wide range of practical applications.
Laser diodes tuned with current, while inexpensive and faster than
thermal methods, generally can suffer from narrow tuning wavelength
spans--which limits practical applications to only time
division-multiplexed (TDM) Bragg sensors. Such lasers are
completely unusable in surface plasmon or semiconductor absorption
edge shift sensors. Broadband light source methods utilize
inexpensive light sources, but use a spectrometer to read the
signals (an optical spectrum analyzer may cost as much as $35,000).
This technique is generally most practical when many sensors are
multiplexed on the same fiber. Still, spectrometers are
temperamental and not well suited to field use. The lasers tuned
with external cavities that are now in use, on the other hand,
typically are more expensive than spectrometers, but have the
advantage of using an inexpensive detector. In addition, such
lasers are typically slow to tune (such as 100 nm/sec) and may be
even more delicate than spectrometers. Scanning (or tuning) speed
is especially important in applications in which absorption and
polarization related noise are significant because of negative
effects on the signal-to-noise ratio (SNR).
In contrast, mass produced MEMS-tunable VCSELs, configured as
sensing instruments, are expected to cost at least an order less
than prior art lasers and be at least two orders of magnitude
faster than prior art lasers.
Surface plasmon resonance-based sensors for biological and/or
chemical monitoring are well known to those skilled in the art.
Generally, surface plasmon waves are electromagnetic waves that may
exist at the boundary between a metal and a dielectric (hereinafter
referred to as the "sample"), Such waves can be excited by light
that has its electric field polarized parallel to the incident
plane (i.e., transverse magnetic (TM) polarized). When the parallel
component of the propagation constant of the incident light equals
the real part of the surface plasmon wave propagation constant, the
incident light resonantly excites the surface plasmon waves, and a
fraction of the incident light energy is transferred or dispersed
to surface plasmon resonance ("SPR"). This dispersion of energy
depends on both the dielectric constant of the metal and that of
the sample in contact with the metal. By monitoring the resonance
wavevector of the metal/sample interface, the dielectric constant
of the sample (gas or solution) may be obtained. Alternatively, if
the sample is contaminated by a chemical species, dielectric
constant measurements may provide the concentration of the chemical
species in the sample. The typical SPR spectral minimum is at least
two orders of magnitude wider than the typical Bragg grating
minimum or maximum.
Traditionally, SPR has been measured using the Kretschmann
configuration (Kretschmann and Raether, Z. Naturforsch. Teil A
23:2135-2136, 1968). In this configuration, a thin layer of highly
reflective metal (such as gold or silver) is deposited on the base
of a prism. The metal surface is then contacted with the sample,
and the SPR reflection spectra of the sample is measured by
coupling TM polarized, monochromatic light into the prism and
measuring the reflected light intensity as a function of the angle
of incidence. The angle of minimum reflective intensity is the
resonance angle at which maximum coupling occurs between the
incident light and the surface plasmon waves. This angle, as well
as the half-width of the resonance spectrum and the intensity at
the angle of minimum reflective intensity, may be used to
characterize or sense the sample that is in contact with the metal
surface (e.g. Fontana et al., Applied Optics 27:3334-3339,
1988).
Optical sensing systems have been constructed based on the
Kretschmann configuration described above. Such systems utilize the
sensitivity of SPR to changes in the refractive indices of both
bulk and thin film samples, as well as to changes in the thickness
of thin films. These systems, in conjunction with appropriate
chemical sensing layers, have led to the development of a variety
of SPR-based chemical sensors, including immunoassay sensors (e.g.,
Liedberg et al., Sensors and Actuators 4:299-304, 1983; Daniels et
al. [Sensors and Actuators 15:11-17, 1988]; Jorgenson et al.,
[IEEE/Engineering Medicine and Biology Society. Proceedings
12:440-442, 1990]), gas sensors (e.g., Liedberg et al., ibid, Gent
et al., [Applied Optics 29:2843-2849, 1990]), and liquid sensors
(e.g., Matsubaru et al., [Applied Optics 27:1160-1163, 1988]). An
SPR sensor usually utilizes the wavelength of minimum amplitude as
a function or angle of reflection. However, the shape of the minima
can be modified if an additional polarizer and phase plate (or
retarder) are introduced between the sensor and detector at some
predetermined angle with respect to the polarization of light
illuminating the SPR sensor (Homola J., et al., Sensors and
Actuators B, B51 (1-3), August 1998, p. 331, Kabashin A. V. et al.,
Sensors and Actuators B, B54 (1-2), January 1999, p.51). This
modification is due to phase and polarization peculiarities near
the surface plasmon resonance excitation conditions. Moreover,
minima can be transformed into maxima (Homola, ibid), which has the
potential for increasing the resolution of SPR sensors
While the Kretschmann configuration for SPR-based chemical sensors
offers significant sensitivity, the relatively large size of the
additional polarizer and phase plate has severely restricted the
application of such arrangements. An optical fiber sensor that
utilizes SPR to detect a material in contact with the sensor and
utilizes incident light having multiple wavelengths as the
excitation energy is described by Jorgenson, et al. (U.S. Pat. No.
5,359,681). While being small and considerably less expensive than
the non-waveguide optical sensor, it is at least an order of
magnitude less sensitive. The reason for this drop in sensitivity
is obvious--in a non-waveguide optical scheme with 5000 pixels, the
SPR minimum is read by at least 2000 pixels. However, in the fiber
optic scheme with a spectrometer as a readout instrument
(wavelength resolution not better than 0.1 nanometer and SPR
minimum spectral width around 60 nm) the SPR minimum will be
characterized by 600 pixels at most, leading to less precise
interpolations to locate the minimum, and hence changes in the
wavelength position of the minimum. Conversely, the use of
inexpensive tunable lasers with sub-nm wavelength resolution as
light sources in fiber systems will eliminate an expensive
spectrometer and yield precision at least comparable to that of the
non-waveguide optical system.
The illustrative problem that arises in the broadband light
source/spectrometer configuration, specifically a lack of optical
intensity per measurement point, has, in the case of SPR sensors,
the additional undesirable attribute of overheating of the sensor.
If the total light intensity of the broadband light source is
increased to compensate for the small intensity available to each
pixel in the spectrometer charge coupled device (CCD) array,
overheating of the SPR sensor will occur because approximately half
the incident light is dissipated in heat in the metal layer whether
it contributes to the usable signal or not. Heating is not only
often harmful to the biological and/or chemical sample under test,
but also can induce refractive index changes in the fiber and/or
sample, causing much larger variations in the SPR wavevector than
the perturbation to be detected. Very fast tuning lasers, having
very narrow emission spectra, are ideal to address this problem,
since the total intensity illuminating the sensor at any given time
will be almost exactly equal to the intensity of the reflected
light available to detect the change in stimulus.
An illustrative example of a characteristic absorber/reflector
material is a semiconductor. A semiconductor-based optical sensor
with optical-wavelength-dependent characteristics that may vary as
a function of a physical parameter such as temperature is well
known to those skilled in the art (see, for example patents, issued
to Christenson (U.S. Pat. No. 4,136,566) or Quick et al. (U.S. Pat.
No. 4,355,910)). The optical-wavelength-dependent characteristic
(semiconductor absorption band edge) is usually monitored in one
wavelength band, in which case measurements are
intensity-dependent, or in two wavelength bands, after which a
ratio of intensities is taken. In both cases, the sensitivity and
accuracy of such sensor systems are low and more-or-less sensitive
to optical noise (microbending, etc.). Scanning very rapidly
through the whole semiconductor transmission intensity slope
related to the forbidden band edge using a tunable VCSEL with high
wavelength resolution will provide the opportunity for mathematical
enhancement of the sensitivity of such sensors by at least an order
of magnitude. Broadband light source/spectrometer configurations
are not suitable for reasons similar to those described above. The
high total illumination intensity may likely cause self-heating of
the sensor, which is detrimental especially for temperature
sensors. Reducing the illumination intensity, on the other hand,
will cause uncertainty due to photodetector dark noise and other
sources of optical noise.
Fiber etalon-based sensors are well known to those skilled in the
art (see, for example, U.S. Pat. No. 5,646,401 issued to E. Udd).
Etalons consist of two mirrored surfaces that may be internal or
external to the optical fiber. The reflectivity of an etalon is
defined by interference between light waves reflected from first
and second mirrors. The advantages of etalon-based pressure,
temperature and/or stain sensors include the low cost of etalons
and very high sensitivity. However, with broadband light sources
used for interrogation, measurements that are intensity based or
count interference fringes are very susceptible to optical noise or
other technical problems (e.g., losing count of the fringes)--often
to the point of being impractical. The sole practical,
self-calibrating system uses an optical cross-correlating
interferometer as a detector, also an expensive technique (see, for
example, U.S. Pat. Nos. 5,202,939 and 5,392,117 both issued to
Belleville, et al.).
A new kind of laser, a vertical cavity surface emitting laser
(VCSEL), has recently been invented. Generally, VCSELs are made
completely with wafer-level processing and the chips emit from the
direction of the broad surface of the wafer, rather than having to
be cleaved out of the wafer in order to have an exposed p-n
junction edge from which to emit, as in older art. This enables
another benefit to be designed into the wafer
structure--tunability. This is done with micromachining (MEMS)
technology by placing a stack of optical layers, forming a mirror,
in front of the emitting surface in such a way that the stack can
be varied in its distance from the emitting surface by
piezoelectric, magnetic, electrostatic or some other microactuating
means.
The groups of C. J. Chang-Hasnain (U.S. Patent, [IEEE J. on
Selected Topics in Quantum Electronics, V 6, N 6, November 2000, p.
978]), J. S. Harris Jr. (U.S. Pat. No. 5291502, [Appl. Phys. Lett.
68 (7), February 1996 p. 891]), and Vakhshoori [Electronics
Letters, May 1999, V. 35, N.11 p. 900] have shown the potential for
making tunable VCSELs with MEMS tuning mechanisms with wide tuning
ranges and fast tuning speeds. Tunable VCSELs are relatively simple
to manufacture, exhibit continuous mode-hop-free tunability over a
wide spectrum, and potentially offer orders of magnitude lower cost
as compared to prior art tunable lasers or optical spectrometers.
Integrated, MEMS-tunable VCSELS make possible truly affordable and
accurate optical sensor systems by combining low cost detectors and
low cost excitation sources, one or the other of which is very
expensive in the prior art systems with the accuracy and resolution
to be viable commercially. In addition to the orders of magnitude
lower cost of source/detector combinations, lower cost sensor will
become available because of the orders of magnitude greater tuning
speed.
Exemplary embodiments of the present invention provide a means of
optical wavelength scanning Bragg grating, characteristic
absorber/reflector, etalon and surface plasmon resonance sensors of
all types with integrated, MEMS-tunable VCSELs in order to measure
various physical parameters at several orders of magnitude lower
cost than prior art, with the added benefits of enhanced accuracy,
ruggedness and reliability.
In more detail, exemplary embodiments of the present invention
provide, as an illustrative embodiment, diagnostic systems which
interface with optical fibers or optical waveguides having Bragg
grating or other types of sensors as described herein, embedded
therein for the determination of static and dynamic values of
various physical, chemical or biological parameters, and, further,
to provide means of guaranteeing wavelength accuracy during the
scanning cycle.
In accordance with one aspect of a preferred illustrative
embodiment of the invention, an optical sensor diagnostic system
includes an integrated MEMS-tunable VCSEL for providing a
wavelength-tunable light in response to a voltage or other control
signal, the tunable light being launched into an optical waveguide.
At least one optical sensor, disposed in the path of the tunable
light, provides a reflected light having an associated local
amplitude minimum, maximum or slope. The said local amplitude
maximum could contain one or more local amplitude minimums inside
said local amplitude maximum, while said local amplitude minimum
could contain one or more local amplitude maximums inside said
local amplitude minimum. The wavelength at said minimum, maximum or
slope of amplitude varies in response to an environmental stimulus
imposed upon the corresponding sensor.
The tunable VCSEL individually illuminates each of the sensors
throughout its associated wavelength band of an amplitude minimum,
maximum or slope. An optical circulation device, disposed in the
path of the tunable light between the tunable VCSEL and the
sensors, isolates the tunable VCSEL from the reflected light and
directs the reflected light from each of the sensors to the optical
detector means, disposed for detecting the reflected light and for
providing an electrical detection signal indicative of the power of
the reflected light. A tuning controller provides a variable
voltage or other signal to the tunable VCSEL indicative of the
desired wavelength of the tunable light. A signal processor
responsive to the electrical detection signal interprets a shift in
the wavelength of the magnitude minimum, maximum or slope due to
the environmental stimulus, and provides a signal indicative of
said stimulus.
According to another aspect provided by an illustrative embodiment
of the present invention, an optical sensor diagnostic system
includes an integrated MEMS-tunable VCSEL for providing a
wavelength-tunable light in response to a voltage or other control
signal, the tunable light being launched into an optical waveguide.
At least one optical sensor, disposed in the path of the tunable
light, provides a transmitted light having an associated local
amplitude minimum, maximum or slope. The said local amplitude
maximum could contain one or more local amplitude minimums inside
said local amplitude maximum, while said local amplitude minimum
could contain one or more local amplitude maximums inside said
local amplitude minimum. The wavelength at said minimum, maximum or
slope of amplitude varies in response to an environmental stimulus
imposed upon the corresponding sensor. The tunable VCSEL
individually illuminates each of the sensors throughout its
associated wavelength band of an amplitude minimum, maximum or
slope. An optical isolation device, disposed in the path of the
tunable light between the tunable VCSEL and the sensors, isolates
the tunable VCSEL from the reflected light. The light transmitted
through the said at least one optical sensor is directed by an
out-going fiber to the optical detector means, disposed for
detecting the transmitted light and for providing an electrical
detection signal indicative of the power of the transmitted light.
A tuning controller provides a variable voltage or other signal to
the tunable VCSEL indicative of the desired wavelength of the
tunable light. A signal processor responsive to the electrical
detection signal interprets a shift in the wavelength of the
magnitude minimum, maximum or slope due to the environmental
stimulus, and provides a signal indicative of said stimulus.
In accordance with one aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of
reflective Bragg grating type. The sensors reflect light, having
maxima or minima inside the maxima at a different reflection
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, electrical current or magnetic field imposed
thereon.
In accordance with another aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
transmittive Bragg grating type. The sensors transmit light, having
minima or maxima inside the minima at a different transmission
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, electrical current or magnetic field imposed
thereon.
In accordance with a further aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
reflective etalon type. The sensors reflect light, having maxima,
minima or maxima and minima at a different reflection wavelength
for each sensor, which vary their spectral positions due to an
environmental stimulus, such as strain, pressure, temperature,
electrical current or magnetic field imposed thereon.
In accordance with a further aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
transmittive etalon type. The sensors transmit light, having
maxima, minima or maxima and minima at a different transmission
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, current or magnetic field imposed thereon.
In accordance with a further aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
reflective Surface Plasmon Resonance type. The sensors reflect
light, having maxima or minima at a different reflection wavelength
for each sensor, which vary in their spectral positions due to an
environmental stimulus, such as temperature, biological or chemical
stimuli imposed thereon.
In accordance with a further aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
transmittive Surface Plasmon Resonance type. The sensors transmit
light, having maxima or minima at a different reflection wavelength
for each sensor, which vary in their spectral positions due to an
environmental stimulus, such as temperature, or biological and
chemical stimuli imposed thereon.
In accordance with a further aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of a
characteristic absorber/reflector type. Said characteristic
absorber/reflector sensors, disposed in the path of the tunable
light, provide a transmitted light having an associated local
amplitude slope or local amplitude minimum, the wavelengths of
which vary their spectral position due to an environmental
stimulus, such as temperature, imposed thereon. This embodiment
could be also realized in reflection mode, with multiple sensors
coupled off the main fiber, if the reflective means is disposed in
the light path such that the light is double-passed through each of
the sensors by means of a mirror and time domain multiplexing. In
the case of an absorber/reflector exhibiting a minimum, wavelength
division multiplexing can be utilized to the degree the width of
the minimum allows as a fraction of the available tuning spectrum.
The isolator in this realization of the present embodiment may be
replaced by a circulator means.
In one example arrangement, the sensor produces a characteristic
absorption feature in the form of a slope, wherein the said
wavelength indicative of the characteristic absorption slope is
determined by taking the first derivative of the light amplitude
with respect to the wavelength, and by analytically extracting the
wavelength position resulting from said first wavelength derivative
extremum. In another example arrangement, the sensor produces a
characteristic absorption feature in the form of a slope, wherein
the said wavelength indicative of the characteristic absorption
slope is determined by taking the second derivative of the light
amplitude with respect to the wavelength, and by analytically
extracting the wavelength positions of said second wavelength
derivative zeros.
The illustrative embodiments of the invention provide low cost,
workable, practical diagnostic systems which function in
cooperation with remote optical fiber sensor systems to measure
static and dynamic strain, pressure, temperature, electrical
currents and magnetic fields as well as acoustic or vibratory
perturbations of items or structures and chemical and biological
parameters. The remote sensors may be disposed on structures made
of metal, plastic, composite, or any other materials that expand,
contract, or vibrate, or the sensors may be embedded within such
structures or immersed in liquids or gasses.
The embodiments also provide a wavelength-tunable VCSEL, tunable
smoothly and monotonically, and in particular, linearly or
sinusoidally tunable with time. The embodiments further provide
individual illumination of each sensor, thereby allowing all the
tunable VCSEL power to be resident in a single narrow wavelength
band at any instant in time. As a result, the reflected or
transmitted light from each optical sensor has a high intensity,
thereby providing a signal-to-noise ratio of such reflected or
transmitted light that is much greater than systems that illuminate
all sensors at the same time using a broadband source.
Ultra-fine tuning of tunable VCSELs to a few parts per million will
allow another order of magnitude increase in precision due to
higher resolution and improved computational methods and
statistical processing. The very low mass of the MEMS tuning
mechanisms allow very high tuning speeds with very low hysteresis,
providing the ability to average out optical noise in the sensor
systems with many data points and allowing very close spacing of
data in wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages will
become more apparent in light of the following brief description of
exemplary embodiments thereof as illustrated in the accompanying
drawings, of which:
FIG. 1 is a schematic drawing of an illustrative VCSEL
incorporating one example of an integrated MEMS
(micro-electromechanical machined system) tuning mechanism in the
form of a cantilevered mirror and optional lens;
FIG. 2A is a schematic block diagram of a first state of an
illustrative, exemplary non-limiting sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological physical stimuli in a reflection mode
employing a tunable VCSEL as an optical excitation source and a
photodiode or similar simple device as a detector;
FIG. 2B is a schematic block diagram of a first state of an
illustrative, exemplary non-limiting sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological stimuli in a transmission mode; employing a
tunable VCSEL as an optical excitation source and a photodiode or
similar simple device as a detector;
FIGS. 3A-3J are a series of illustrative, exemplary graphs showing
time-varying tuning control signals, V.sub.t, applied to a tunable
VCSEL and the resulting spectral power transmission and/or
reflection signals produced by the sensor;
FIG. 4 is a graph of an illustrative, exemplary transmission
optical power profile of a sensor employing Surface Plasmon
Resonance to produce a shift in the wavelength position of the
spectral power minimum in response to a physical, chemical or
biological stimulus;
FIG. 5A is an illustrative, exemplary diagram of the spectral power
transmission of a characteristic absorber/reflector in the form of
a bandgap semiconductor showing a shift in wavelength due to an
increase in temperature; and
FIGS. 5B and 5C are, respectively, illustrations of the increases
in the accuracy of wavelength shift determination with the
knowledge of the first and second derivatives of the power
transmission spectra.
DETAILED DESCRIPTION
FIG. 1 is an illustrative schematic drawing of a VCSEL
incorporating one example of an integrated MEMS
(micro-electromechanical machined system) tuning mechanism in the
form of a cantilevered mirror and optional lens. Substrate chip 30
has fabricated upon it, when in wafer form, a multilayer stack of
materials forming the light emitting VCSEL 31 and the tuning
components consisting of mirror stack 32, actuator and structural
means 33 that change the tuning cavity length 38, diffractive
optical lens 34 (optional), capacitive cantilever position monitor
35 (optional) and light beams emitted in directions 36 and/or 37.
If either mirror 32 or bottom of stack 31 is opaque, light is
emitted only in one direction 37 or 36 respectively. Emission in
both directions is possible if both 32 and 31 are partially
transparent. This arrangement provides a simple means for optical
power monitoring with a photodiode 38 for the purpose of spectral
power uniformity control. Electrical connections are not shown for
simplicity.
FIG. 2A shows an example illustrative embodiment of a diagnostic
system 40. In more detail, FIG. 2A is a schematic block diagram of
a first state of an illustrative exemplary sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological physical stimuli in a reflection mode
employing a tunable VCSEL as an optical excitation source and a
photodiode or other optical, capacitive, piezoelectric and/or
magnetic device or similar simple device as a detector. Preferred
embodiment diagnostic system 40 includes a MEMS-tunable VCSEL 30, a
fiber light coupling means 41, an optical circulator 47, a
wavelength reference 43, exterior fiber 44 and coupling means to
the sensor or sensor array 45, a photodetector 48, and a control
block 49. An optical amplifier may be disposed between VCSEL 30 and
sensor 48 or it may be part of the VCSEL. The amplifier may
comprise e.g. an Erbium doped fiber amplifier or a semiconductor
amplifier or other arrangement.
In said illustrative example, a tunable VCSEL 30 is assembled with
necessary means to fiber couple 41 the emitted light, provide an
accurate wavelength reference 43, and couple the VCSEL assembly 40
to an external fiber 44 to convey the laser light to an optical
sensor or sensor array 45 in a reflection mode. A coupler or
circulator 47 is provided to divert the optical signal reflected
from the sensor 45 to the photodetector 48, the electrical signal
from which is relayed to the control block circuitry 49 and
external electronic circuitry as desired. A circulator also
provides the function of isolating the VCSEL from back-reflected
light. If a coupler is used to divert the light to the detector, a
separate isolator can be incorporated between it and the laser. The
control block 49 may or may not control the laser temperature via a
thermoelectric element or other means and may or may not adjust the
laser power output according to a signal from a monitor photodiode
38, as desired.
In this embodiment, the tunable VCSEL 30 provides a
wavelength-tunable light in response to a tuner control signal
provided by control block 49. This tunable light provided by
tunable VCSEL 30 is launched into an optical waveguide 44 such as
an optical fiber. A sensor or sensor array 45, in this embodiment a
Bragg grating sensor array, providing at least one optical sensor,
is disposed in the path of the tunable light. The sensor array 45
includes individual Bragg gratings that each reflect light having
different, non-overlapping, associated amplitude reflection maxima
at individual reflection wavelengths, spectrally distinguishable
one from the other. In the exemplary embodiment, the wavelength
position of the amplitude maximum reflected by each of the Bragg
gratings in the array 45 varies in response to a physical stimulus
or perturbation imposed on the corresponding sensor. The Bragg
gratings may for example be fiber or planar Bragg gratings, narrow
bandpass filter and/or a long period fiber Bragg grating having a
period exceeding e.g., 10 .mu.m.
The tunable VCSEL 30, by continuously scanning its output spectrum,
individually illuminates each of the sensors in turn within the
sensor array 45 in a wavelength band including the wavelength of
maximum or minimum reflection associated with each sensor. An
optical isolation and directing device such as optical circulator
47 is disposed in the path of the tunable light between the tunable
VCSEL 30 and the sensor array 45. The circulator efficiently
isolates the tunable VCSEL from light reflected by the sensor array
45 and diverts the reflected signals to a simple and inexpensive
optical detector 48, such as a photodiode, disposed in the path of
the light. The detector 48 provides an electrical detection signal
indicative of the power of the reflected light that is directly
related to the wavelength through the tuning control signal and the
wavelength reference 43, if utilized. The series of optical signals
obtained during a scanning cycle can contain one or more absorption
or reflection bands from one or more wavelength reference devices
43 that can be incorporated for additional wavelength accuracy.
Said reference signals do not change in wavelength position with
any of the external stimuli measured by the sensors, and can be
related in time to the tuning control signal.
Control block 49 responds to the electrical detection signal from
the photodetector 48 in the example embodiment by calibrating a
variable voltage or other tuning signal for the tunable VCSEL 30 to
the wavelengths of the wavelength references, and providing said
tuning signal to said VCSEL. Control block 49 may also include a
signal processor responsive to the electrical detection signal for
detecting a shift in the wavelength of maximum reflection due to a
physical, chemical or biological stimulus on each of the sensors,
and/or may cooperate with external circuitry to provide a signal
indicative of the stimulus for each of the sensors. Control block
49 may also control the laser temperature by any of several known
means and adjust the laser power to provide a constant power output
with respect to wavelength using an independent monitor detector 38
(FIG. 1).
In more detail, referring to FIG. 2A, diagnostic system 40 includes
a tunable VCSEL 30 which in this embodiment (FIG. 1) has a rear
reflector stack of alternating quarter-wave layers of two different
materials 31, the Fabry-Perot cavity region that contains the
active material 31 (here a solid optical cavity), and an upper
reflector 32, made as movable, suspended mirror layers with
different indices of refraction of transparent material on a
cantilever as illustrated, or, alternately, as a reflective or
partially reflective single layer, such as aluminum. The relative
position of the movable mirror structure with respect to the rest
of the structure is changeable by the application of an
electrostatic field or other control force, forming a variable
optical cavity 38 (here an air or vacuum optical cavity). The
mirror structure could be made in a form of a diaphragm suspended
by other means by selective etching and release techniques, the
relative position of which with respect to the rest of the
structure is also changeable by the application of an electrostatic
force, magnetic force or other force. The result of this is that
the effective optical distance between the two reflectors making up
the cavity 38 is adjustable. Since the resonant wavelength depends
on this distance, the characteristic wavelength of the tunable
VCSEL is continually tuned, for example, by varying the applied
voltage and thereby the electrostatic field between the upper
reflector and the remainder of the device.
It is desirable to provide energy within the tunable VCSEL 30 to
achieve lasing. It should be noted that the energy could be
provided by optical pumping means or by electrical pumping means
(p-n or p-i-n junction). Although both methods are suitable for a
sensor system, the electrically pumped embodiment is preferred from
the point of view of lowest cost and greatest simplicity.
The operating wavelengths of the tunable VCSEL can be in the
communication wavelength band (Chang-Hasnain [IEEE J. on Select.
Topics in Quantum Electronics, V 6, N 6, November 2000, p. 978]
Vkhshoori [Electronics Letters, May 1999, V. 35, N.11 p. 900]) or
around 960 nm (J. S. Harris, [Appl. Phys. Lett. 68 (7), February
1996 p. 891]) or in any other desired band in which VCSELs are
produced. When the distance between the tunable VCSEL 30 and a
Bragg grating or other type of sensor 45 does not exceed about 1
km, many wavelength bands are usable. When this distance exceeds
about 1 km, the losses may become too high at wavelengths not in
the communications bands and tunable VCSELs 30 emitting within the
communication wavelength bands may be more suitable.
A current control circuit within control block 49 (FIG. 2A)
provides an electrical current to the tunable VCSEL 30, which
controls the intensity of the output light. Adjusting the current
through the diode (VCSEL active area 31) also causes slight changes
in wavelength. However, this effect is not significant for this
application. A pulsed current can be used to cause pulsed light,
which would be suitable for Time Division Multiplexing (TDM)
(although it should be noted that TDM could be realized by placing
an electro-optical modulator anywhere between tunable VCSEL 30 and
sensor array 45). In addition, a temperature control circuit could
be used in the illustrative embodiment to provide a current drive
to a thermoelectric (TE) cooler to stabilize the temperature of the
tunable VCSEL 30 if needed. Other devices may be used to control
the temperature if desired. A voltage control circuit can be used
to control the electrostatic force between the movable reflector 32
and the active layers of the tunable VCSEL 31 and, by such means,
can control the wavelength emitted by the tunable VCSEL in the
illustrative embodiment. It should be noted that control mechanisms
other than electrostatic can be used to position the VCSEL tuning
mirror, and the tuning signal may or may not be a voltage.
In all exemplary embodiments, the tunable VCSEL 30 can provide a
divergent output light beam to either the end plane of fiber 41,
placed in close vicinity to the tunable VCSEL and perpendicular to
the direction of emitted light propagation (butt-coupling method)
or to a focusing lens, also represented by element 41, that
provides focused light to optical fiber component isolator 42 or
circulator 47. The lens may instead be a lens system that provides
this function. The lens also could be realized as a diffractive
clement 34 written photolithographically on the surface of the
VCSEL mirror, adjacent to the fiber 41 or on the backside of the
chip in the path of the light beam 36.
It should be noted that the optical circulator 47 may be replaced
by an optical isolator 42 and a wavelength-independent two-way
splitter, placed in line. This approach is less costly, although at
least half of the optical power will be lost going each way. In
addition, an optical isolator could be placed between the tunable
VCSEL 30 and the optical circulator 47 if very high suppression of
back-reflected light is needed. In this embodiment, the sensor can
be a fiber Bragg grating, a planar Bragg grating, a surface plasmon
resonance sensor, a Fabry-Perot etalon sensor or a characteristic
absorber/reflector sensor.
Additionally in the FIG. 2A embodiment, the light from the tunable
VCSEL 30 propagates toward the sensor 45 that is composed of an
array of sensors disposed at intervals along the optical fiber 44.
Each Bragg grating sensor within sensor array 45 reflects a
predetermined narrow wavelength band of light and passes the
remaining wavelengths on toward the next sensor. The transmitted
beam therefore contains a narrow absorption band corresponding to
the reflected band, but the remainder of the light is available to
use with the other sensors. The sensors in array 45, in the
illustrative embodiment, can be placed in parallel or in series and
multiplexed by Wavelength Division Multiplexing (WDM) and/or TDM.
WDM is realized by sensors 45 having different central reflection
wavelengths, while TDM is realized by intentionally introduced time
delays, by any technique known to those skilled in the art, between
sensors that may or may not have the same central reflection
wavelengths. In this embodiment, the sensors can be fiber Bragg
gratings, planar Bragg gratings, surface plasmon resonance sensors,
characteristic absorption/reflection sensors or Fabry-Perot etalon
sensors.
FIG. 2B shows an example illustrative embodiment of a diagnostic
system 40 provided in accordance with a second aspect of this
invention. FIG. 2B is a schematic block diagram of a first state of
an illustrative, exemplary sensor diagnostic system capable of
determining the value of static and dynamic physical, chemical or
biological stimuli in a transmission mode; necessarily employing a
tunable VCSEL as an optical excitation source and a photodiode or
similar simple device as a detector. Preferred embodiment
diagnostic system 40 includes a MEMS-tunable VCSEL 30, a fiber
light coupling means 41, an optical isolator 42, a wavelength
reference 43, exterior fiber 44 and coupling means to the sensor or
sensor array 45, exit fiber 46, a photodetector 48, and a control
block 49. In said illustrative example, a tunable VCSEL 30 is
assembled with necessary means to fiber couple 41 the emitted
light, isolate 42 the VCSEL from back-reflections, provide an
accurate wavelength reference 43 and couple the VCSEL assembly 40
to an external fiber 44 to convey the laser light to an optical
sensor or sensor array 45 in a transmission mode. An exit optical
fiber 46 is provided to couple the optical output transmitted
through the sensor or sensor array 45 to the photodetector 48, the
electrical signal from which is relayed to the control block
circuitry 49 and external electronic circuitry as required. The
control block may or may not control the laser temperature via a
thermoelectric element or other means and may or may not adjust the
laser power output according to a signal from a monitor photodiode
38, as required.
In this embodiment, the sensor array 45 includes individual Bragg
gratings that each transmits light having different,
non-overlapping, associated amplitude transmission minima at
individual transmission wavelengths, spectrally distinguishable one
from the other. In the exemplary embodiment, the wavelength
position of the amplitude minimum transmitted by each of the Bragg
gratings in the array 45 varies in response to a physical stimulus
or perturbation imposed on the corresponding sensor. The sensors in
array 45, in the illustrative embodiment, can be placed in parallel
or in series and multiplexed by Wavelength Division Multiplexing
(WDM). TDM is not applicable in this embodiment. Other aspects of
this embodiment are the same as in the first embodiment, drawn in
FIG. 2A. In this embodiment, the sensors can be fiber Bragg
gratings, planar Bragg gratings, surface plasmon resonance sensors,
characteristic absorption/reflection sensors or Fabry-Perot etalon
sensors.
In preferred embodiments illustrated in FIGS. 2A and 2B, the fiber
44 and the sensor array 45 may be bonded to or embedded in a
structure which is being monitored for a perturbation change, such
as dynamic or static strain and/or temperature and/or pressure
and/or electrical current/or magnetic field. The structure may be
made of metal, plastic, composite, or any other materials and the
sensors may be disposed on or within the structure.
Signal processing circuits (FIGS. 2A, 2B) analyze the electrical
signals and provide a plurality of output electrical perturbation
signals, indicative of the perturbation being measured by the
sensors within the structure. It should be understood that a single
line that is time multiplexed or that provides serial digital data
for each sensor might also be used.
In the embodiments illustrated in FIGS. 2A and 2B, the wavelength
tuning control circuitry in control block 49 may include a function
generator in order to produce the control signal waveforms
illustrated in FIGS. 3A-3J. FIGS. 3A-3J are a series of graphs
showing exemplary, illustrative time-varying tuning control
signals, represented by V.sub.t, applied to a tunable VCSEL 30.
Output wavelengths, .lambda., and the resulting optical power
spectrum from the sensors 45 as a function of both time and
wavelength in reflection and transmission modes are shown as well.
The waveforms shown are a sawtooth waveform (3A-3D), sinusoidal
waveform (3E-3H) and triangular waveform (3I, 3J), but many others
could be used. It is important to the sensor system operation that
the wavelength versus time should be known accurately, and the
linear triangle wave (3I, 3J) would be superior from that point of
view. The triangle waveform also allows reading all sensors 45
twice per cycle. In the exemplary embodiment, the control signal
V.sub.t relates directly to the expansion or contraction of the
cavity 38 in the VCSEL 30, thereby causing the wavelength .lambda.
of the output light to vary in proportion to the applied control
signal V.sub.t. Thus, the wavelength .lambda. of the light varies
linearly from .lambda..sub.1 to .lambda..sub.2, which range
includes at least one peak reflection or transmission minimum
wavelength .lambda..sub.b from a sensor and optionally at least one
peak or minimum from a wavelength reference, which is shown as WR.
For the sake of simplicity of the figures, reflection or
transmission from just one sensor is shown. Also for simplicity of
illustration, the optical power graphs for the triangular wave are
not shown, as they are similar to 3C and 3D with the sensor signals
occurring twice per cycle.
The triangle waveform (FIGS. 3I, 3J), although providing linear
dependence of the wavelength vs. time, has the disadvantage of
having a discontinuity in the waveform that will by its nature
induce higher frequencies, or ringing, into the system. The ringing
can be filtered out by various means known in the art, but a
penalty is paid in time and efficiency. The sinusoidal control
signal will provide frequency stability and power-conserving
scanning with much faster scanning rate due to the elimination of
the stabilizing time required of a mechanical structure when a
discontinuous forcing function is applied, such as the triangle
wave. With the sinusoidal waveform, the entire scan can occur in a
few microseconds or shorter time. This is two orders of magnitude
speed advantage over conventional lasers, allowing better
statistical averaging techniques to be used and allowing
non-spectral shift sensors (e.g., bandgap semiconductor or other
characteristic absorber/reflectors) to be scanned fast enough to
minimize optical noise interference. The optical power sensor
signals for the sinusoidal waveforms are illustrated schematically
in FIGS. 3G and 3H, showing the non-linear nature of the signals in
time.
As a result of the scan through the wavelength range, the optical
signal at the input to the optical detector 48 as well as the
electrical signal from the optical detector will appear as
indicated in illustration FIG. 3C or 3G for the reflection mode and
FIG. 3D or 3H for the transmission mode. In particular, the
electrical signal from the optical detector will experience a sharp
increase or decrease respectively (as dictated by sensor design) in
power centered at the central wavelength .lambda..sub.a of each
sensor. If TDM is also used, the output is somewhat more complex,
but known to those skilled in the art. For the sinusoidal drive,
the results of a scan are given in FIGS. 3G, 3H.
In the illustrative embodiment, the control block 49, in
cooperation with additional signal processing circuitry, determines
the static or dynamic value of the sensor stimulus by determining
at what wavelengths the maxima or minima in signal level occur and
determining the amount of change from the wavelength maximal or
minima of the unperturbed sensors. Calibration defines the
relationship between a change in the stimulating parameter and a
corresponding change in wavelength. The wavelength value is
determined by monitoring the wavelength control signal and
comparing it to the wavelength reference 43 or mirror 32 position
feedback 35 (capacitive), as required. Because this signal is
directly related to the wavelength of the tunable VCSEL 30, it
provides a directly proportional value of the instantaneous
wavelength. Many computation algorithms known to those skilled in
the art can perform the determination of wavelength position of the
minima or maxima. For the illustrative purposes only, one of the
possible algorithms is described below. The ability to calculate
the position of an extremum from relatively few data points enables
enhanced accuracy with lower computational overhead.
During each full tunable VCSEL wavelength scan, N intensity
measurement points are taken. Since photodetector 48 and the signal
processor in the control block 49 can be made to operate by known
art at GHz frequencies, the number N could be adequately large even
if the tunable VCSEL could be operated at a maximum tuning speed of
tens of kHz. Let us assume that the tunable VCSEL operates at a
frequency of only 1 kHz (each scan takes 1 millisecond), which is
still two orders of magnitude faster than commonly available from
non-MEMS tunable lasers. In this case, at least 100,000 intensity
measurements could be taken during each scan. Further assuming the
maximum wavelength span of the tunable VCSEL does not exceed 50 nm,
intensity measurements could be made every 0.5 picometers. Since
communications art has advanced into the tens of GHz and slower
scanning speeds can be tolerated in practical situations, greater
wavelength resolution could be obtained. Further, since typical
Bragg reflection peaks (or transmission minima) wavelength widths
are on the order of hundreds of picometers, the embodiments
illustrated in FIGS. 2A and 2B are capable of extremely high
wavelength resolution. In the embodiment of Bragg gratings
employing phase shifts, in which a much narrower peak or valley (20
picometers or narrower) is incorporated inside the primary valley
or peak (respectively), the sensor data rate obtainable, as
illustrated, will enable several data points to be taken within the
phase shift band. This in turn will allow the interpolation of the
spectral data from the sensor array by a mathematically smooth,
continuous function of time, F(t). F(t) can then be transformed to
a function of wavelength, F(.lambda.), according to the type of
VCSEL tuning drive used, or directly into a function of the
parameter being measured. Many applicable mathematical techniques
and their electronic implementations are known in the art. When
used, wavelength references can be analyzed in the same manner.
Further simple algorithms are used to compute wavelength changes
for each sensor by comparison to the previous F(.lambda..sub.s) and
the wavelength reference.
Instead of relying on the tuning control signal or feedback from a
cantilever or diaphragm position monitoring means, such as
capacitance, to calibrate the VCSEL wavelength against time, an
additional unstrained or unperturbed reference means in the form of
at least one Bragg grating, Fabry-Perot etalon or absorption cell
may be inserted into the optical path at 43. Said reference grating
or cell causes at least one reflection peak or absorption valley
within the tuning range and does not interfere with any sensor
wavelength band, and may provide multiple extrema at
.lambda..sub.ref 1, .lambda..sub.ref 2, .lambda..sub.ref 3, . . .
.lambda..sub.ref n that are always located at the same wavelength
positions. Knowledge of the predetermined cycle rate, or waveform,
of the voltage or other tuning signal, together with such reference
wavelengths, provides the signal processing circuit with sufficient
information to synchronize the beginning of each new tuning cycle
with the laser wavelength. The number of wavelength reference
points is determined by the accuracy and linearity of the laser
tuning mechanism and the required accuracy of the physical
parameter measurement. The fewest reference points will generally
provide the most economical system. In place of a reference Bragg
grating or gratings, a number of high finesse Fabry-Perot cavity
filters could be used. Another applicable method of maintaining
wavelength accuracy would be to place an acetylene cell in the
optical path. Acetylene exhibits a number of very sharp absorption
peaks in the communications wavelength bands that can be used to
calibrate the system on every cycle or every half cycle. Other
techniques may also be employed to maintain calibration accuracy to
needed levels by those skilled in the art.
Even though the embodiments have been most frequently described as
using Bragg gratings as the sensors that detect the environmental
stimulus, any reflective or transmittive device having a narrow
reflection or transmission wavelength band, or transition slope
(e.g., bandgap semiconductors) or any other reflection or
transmission spectral peculiarity that shifts with applied
perturbation may be used. Some examples of such sensors include
Fabry-Perot cavity pressure, temperature and/or displacement
sensors, waveguide and surface plasmon resonance-based biological
and/or chemical sensors, semiconductor bandgap strain, temperature
or pressure sensors and fluorescent and vibronic materials. In the
latter two types of materials, the absorption bands are much
narrower than that of semiconductors, and they do not have to
exhibit fluorescent light output in the spectral range of use.
An example of a surface plasmon resonance-based sensor transmission
spectrum is given in FIG. 4. The valley in the curve, caused by a
surface plasmon, is shifted in wavelength by, for example, a
biomass specimen to be detected as it is adsorbed onto the sensor
surface. The absolute value of said shift provides very precise
information about the concentration of, for example, a reagent in
the solution. Additionally, the temporal behavior of said shift can
provide information about the kinetics of associated chemical
interactions. The high tuning rate of the MEMS-tunable VCSEL
enables faster and higher resolution kinetics studies possible,
widening the range of applications. The employment of MEMS-tunable
VCSEL 30 as a light source will make the afore-described types of
sensor systems considerably less expensive and more functional than
those employing prior art lasers and/or optical spectrometers and
will provide at least an order of magnitude increase in the system
resolution through computational and statistical means. Another
feature of surface plasmon resonance sensors is that the resonance
transmission valley can be located at any predetermined wavelength
within the near infrared or infrared spectrum. Thus, tunable VCSELs
30 operated at 950-980 nm, for example, will be equally as useful
as VCSELs operating in the communications bands in the 1310 nm and
1550 nm ranges, with little change in cost. This further advantage
of tunable VCSEL sensor systems allows a wide variety of sensor
types and materials to be matched with a suitable, inexpensive
tunable laser. The mathematical algorithm for extracting the
position of the wavelength at the minimum of transmission will be
similar to the one described for the reflective or tranmissive
Bragg grating sensor above. Wavelength multiplexing in this case
will be limited by the greater width (typically tens of nanometers)
of SPR reflectivity minima and the maximum tuning wavelength span
of the tunable VCSEL, possibly as low as two sensors. Time division
multiplexing is applicable in this case, as well.
An illustrative example of the transmission spectrum of a
semiconductor bandgap absorption-edge temperature sensor is shown
in FIG. 5. The bandgap edge wavelength position is a function of
the semiconductor temperature according to well-known mathematical
and physical formulations. Such an absorption edge is blue-shifted
as a whole when the temperature decreases and red-shifted when the
temperature increases, as illustrated in FIG. 5A. In this case,
which is illustrative of its type, the scanned absorption edge will
only yield accurate temperature or pressure data if the scanning
process is completed in a much faster time than either the thermal
response time of the semiconductor mass or the speed and/or the
frequency of absorption-dependent noise (e.g., microbending noise)
in the remainder of the fiber circuit. This is because the
absorption of the semiconductor cannot be distinguished from
absorption noise unless the shape of the edge can be traced out
very rapidly. For the greatest accuracy, the spectral shape,
undistorted by optical noise, of the absorption curve is required,
not the absolute value of the absorption. Thus an advantage of the
exceptional tuning speed of the MEMs-tuned VCSEL 30 in this
arrangement will make possible inexpensive fiber optic temperature
sensors using chips of many semiconductor materials, with or
without a mirrored surface, with good sensitivity and adequate
accuracy for applications such as microwave ovens. Further, such
sensors can be selected to match the desired temperature range and
VCSEL properties by using alloy compositions available with
continuously varying bandgaps, such as alloys and compounds of
indium, aluminum, gallium and arsenic or silicon and germanium.
FIGS. 5B and 5C are illustrative examples of improved means of
detecting precisely the spectral position of an absorption band
with a very wide maximum, such as the "long pass" band of a
semiconductor. The "S"-shaped absorption curve can be converted to
a peaked curve by taking the first derivative, as shown in FIG. 5B.
The computational algorithm for this case is then similar to the
cases of Bragg grating sensors and SPR-based sensors described
heretofore. This is equally applicable to a single pass of the
light through the absorber in transmission, or a double pass
through the material if a mirror means is utilized on the output
side of the sensor. A second means of accurately determining the
amount of wavelength change from the unperturbed wavelength is
provided by taking the second derivative of the transmission
spectrum, as shown in FIG. 5C, which provides the opportunity to
use a "zero cross-over" point to define the spectral shift. In the
case of a pure semiconductor, the sensors will be self-calibrating
since the wavelength dependence of the absorption edge is very well
known. In the case of alloys, (e.g., characteristic absorber
material is chosen from the group consisting of alloys and
compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead,
aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus,
arsenic, antimony, sulfur, selenium and tellurium) calibration may
be performed. Many computation algorithms known to those skilled in
the art can perform the determination of wavelength position
utilizing the peak of the first derivative or the zero crossing of
the second derivative. The spectra can be interpolated, smoothed or
subjected to any other mathematical analysis known to those skilled
in the art.
Referring to Bragg grating sensors, the sensors 45 need not be
written into the same type of fiber 44 as the fiber that feeds the
sensors, e.g., the sensors can be splice into the fiber 44 or they
can be separate planar chips, optically coupled to the fiber by
means commonly known in the art.
Further, the embodiments have been described as employing an
optical fiber 44, but any other form of optical waveguide may be
used if desired.
Also, it should be understood that the tuning control circuit 49
and subsequent signal processing can be done with any degree of
combination of software and hardware by many methods known in the
art.
It should be understood by those skilled in the art that the
foregoing and various other changes, omissions and additions may be
made without departing from of the invention.
* * * * *
References